It is known in the prior art to produce printing plates from acrylic elastomer resins and synthetic rubbers utilizing photoengraving and chemical milling techniques. Generally, a layer of such resins or rubbers are placed onto a surface of a flexible plate and then exposed to light through a photographic film or stencil having a desired pattern to be formed onto the printing plate. Exposure of the resins and rubbers to light cures the resins and rubbers, altering the susceptibility of the resins and rubbers to removal by a photopolymer solvent. By applying the photopolymer solvent to the light exposed layer of resins or rubbers on the printing plate, the uncured photopolymer resin or rubber is chemically etched away by the photopolymer solvent in a pattern reverse to that of the film or stencil. The resin or rubber etched from the plate is removed with the waste photopolymer fluid.
Typically, the solvents utilized for the photopolymer solvent are relatively costly and generally have flash point temperatures above 140° F. Further, disposal of the waste photopolymer fluid is likewise relatively expensive and presents environmental problems and concerns. Therefore, to more efficiently produce the aforementioned printing plates, devices were developed to recover the photopolymer solvent from the waste photopolymer fluid.
Prior art photopolymer solvent recovery apparatus utilize vacuum assisted distillation typically within a still to separate the photopolymer solvent from the photopolymer resins or rubbers. Upon separation and removal of the photopolymer solvent, the residue resins or rubbers remain in the device as a concentrated residue. Typically, the residue is removed from the device by gravity flow through a drain. Due to the relative cost of the photopolymer solvent, the operator desires to maximize solvent recovery, however, the recovery of the photopolymer solvent is limited. Over a period of time, concentrated resins or rubbers can coalesce into an amorphous solid residue and cake onto internal surfaces and parts of the still. Once the solid is formed, the apparatus can not be operated until the operator enters and manually removes the solid from the device. Further, because the distillation process involves cyclic heating, it is desirable to periodically inspect the internal structure and parts of the distillation device for signs of structural fatigue. Generally, internal access to the distillation device by the operator is gained through a manhole which utilizes a removable closure.
In the photopolymer solvent recovery industry, recent innovations have been primarily directed to maintaining residue fluidity while attempting to maximize photopolymer solvent recovery. However, very little attention has been given to the manhole and closure of the distillation apparatus. Typically, manhole and closure assemblies consist of a manually adjustable threaded closure which releasably engages mating threads located around the perimeter of a rim extending upwardly proximate the manhole. The fastener assemblies secure the closure on the distillation apparatus and seal the interior thereof. A center speed ball handle lifts the closure from the apparatus, with the fasteners disconnected, and swings away from the open top of the tank on a davit arm. An example of such a manhole and closure assembly is described in U.S. Pat. No. 5,308,452 to Marks et al.
In our pending U.S. patent application Ser. No. 09/151,222 filed Sep. 10, 1998, entitled “Modular Solvent Recovery Device”, a manhole device is described which removably covers and seals a manhole of a still which can be quickly and easily engaged by a single operator. The manhole device comprises a pivotally and telescopically mounted closure which removably engages an annular flange extending outwardly from the still at the manhole. Wheels rotatably mounted to the closure engage tracks, which are positioned on the still, to raise the closure vertically above the manhole upon pivotal movement of the closure. The tracks have rounded or tapered shoulders at their proximal ends to immediately engage the wheels upon initiation of pivotal movement of the closure. As the closure pivots by a relatively nominal physical effort by the operator, the wheels engage the respective shoulders and cause the closure to disengage and vertically rise above the flange. As the closure continues to pivot, the wheels roll along the tracks to clear the closure from obstructing the manhole. A stop mounted to at least one of the tracks proximate its distal end engages the closure to terminate pivotal movement beyond the stop. A foam-covered handle extends outwardly from the closure to assist the operator in pivoting and raising the closure. To assist in sealing the closure to the flange, a gasket is disposed between the closure and the rim. Clamps are pivotally mounted to the still adjacent the flange to releasably engage the closure and contract the closure into sealable engagement with the flange. By pivoting the closure away from the stop, the wheels roll upon and disengage the respective tracks at the shoulders, thereby permitting the closure to drop and seat on the flange and form a sealable engagement with the manhole upon clamping. Unfortunately, because the gasket must rest upon the flange to form the seal, the gasket is not secure from movement along the flange upon pivotal action of the closure. This can result in a poor seal and/or a damaged gasket. In an effort to secure the gasket from movement, the closure can be provided with an annular groove disposed adjacent a periphery thereof to receive the gasket. However, the gasket can slip downwardly or even fall from the groove due to gravitational forces, particularly when the gasket is pliant due to heating, and again result in a poor seal and/or damaged gasket. Further, because the gasket must rest upon the flange, a costly endless annular gasket is generally required to form the seal.
Despite existing manhole and closure assemblies, the need remains for a sealable lid for a vacuum distillation apparatus which secures a gasket from movement and removably covers and seals a manhole of the distillation apparatus or other vessel which can be quickly and easily released from, moved about, and engaged with the manhole by nominal physical effort of a single operator. Further, there remains a need for such a device which permits utilization of a non-endless gasket. Accordingly, it is to the provision of a sealable lid for a vacuum distillation apparatus that meets these needs that the present invention is primarily directed.